The invention relates to a high-pressure sodium discharge lamp provided with a discharge vessel with a ceramic wall of aluminum oxide in which at least Na as an ionizable filling component, a rare gas and Al are present, and provided with main electrodes between which a discharge takes place in the operational condition.
A lamp of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 4,859,905. In the known lamp, the At is fastened to an electrode rod in the form of a Zr-Al alloy. The At then functions as a getter for free O.sub.2 which is evolved or present as an impurity in the discharge vessel. This is important because free O.sub.2 in the discharge vessel leads to the formation of sodium aluminate via the intermediary of NaO, so that Na as an ionizable filling component disappears. It is of great importance to counteract Na-disappearance in all those cases in which the Na is present in the discharge vessel in only a small excess quantity or even not in excess at all. However, besides Na-disappearance owing to free O.sub.2, Na-disappearance owing to direct chemical reactions between Na and wall material Al.sub.2 O.sub.3 takes place as well, leading to the formation of sodium aluminate. The measure taken in the known lamp, however, has no noticeable influence on these forms of Na-disappearance.